The present invention relates to a composition and method for cleaning automatic mechanical dishwashers, and particularly for removing mineral deposits from dishwasher interiors, and maintaining the interior surfaces of dishwashers free of such mineral deposits.
The typical dishwasher cycle consists of one or more initial rinsing cycles followed by a washing cycle, and then further followed by additional rinsing cycles to remove detergent from the dishes. The dishwasher is customarily attached to a hot water supply so that the rinse cycles are carried out with warm to hot water at a temperature usually less than 120xc2x0 F., i.e. about 49xc2x0 C. The wash cycle is normally carried out at a temperature of about 160xc2x0 F., i.e. about 71xc2x0 C., or greater.
Dishwasher interiors tend to develop deposits of white and gray materials caused by the deposition of insoluble minerals from the water supply, particularly in xe2x80x9chardxe2x80x9d water areas. These minerals usually include at least calcium carbonate, but often include carbonates and/or oxides of magnesium, iron and other insolubles. Insofar as is known, the aforementioned problem has not been solved.
While some xe2x80x9cdishwasher detergentsxe2x80x9d, i.e. detergents used in automatic dishwashers for washing dishes, may help prevent the aforementioned deposits during the so-called xe2x80x9cwashxe2x80x9d cycle, the problem which exists is that automatic dishwashers invariably go through a series of rinse cycles following the wash cycle, and the aforementioned deposits build up from repeated rinse cycles over days, weeks and months. Even those dishwasher detergents which may tend to inhibit deposition of minerals during the wash cycle are not very effective in this regard and in any event do not serve to wash away such minerals which have already been deposited from previous cycles. Moreover, most dishwasher detergents are employed at a neutral to alkaline pH, which is not conducive to removal of mineral deposits.
Altenschopfer et al U.S. Pat. No. 4,465,612 relates to a product for cleaning and maintaining the interior surfaces of a mechanical dishwasher, but this is a liquid product and therefore is dissipated and washed away during the first cycle of the machine, usually a rinse cycle preceding the wash cycle. The cleaning liquid disclosed in the Altenschopfer U.S. Pat. No. ""612 is preferably scrubbed onto the interior surface of the dishwasher, and subsequently wiped off, before then running the dishwasher.
Another liquid product advertised as cleaning dishwashers is a product called xe2x80x9cDishwasher Magicxe2x80x9d(trademark) which also is a liquid product (see dishwashermagic.com).
Chelating agents such as EDTA and others are known to be useful for the removal of mineral deposits, often called xe2x80x9cscalexe2x80x9d, including calcium carbonate deposits, from a variety of surfaces including pipes, heat exchangers, evaporators, filters, swimming pools and even false teeth, noting for example U.S. Pat. Nos. 5,972,868; 5,492,629; 5,486,304 and 3,956,164. The contents of these documents are incorporated by reference, insofar as they are consistent with the requirements of the present invention as described below.
According to the present invention, there is provided a solid dishwasher cleaner in cast or compressed tablet form which is adapted to dissolve only slightly during the initial rinse cycles at temperatures less than 110xc2x0 to 120xc2x0 F. (about 43xc2x0 C. to about 49xc2x0 C.), to then dissolve more completely during the wash cycle at temperatures greater than 110xc2x0 to 120xc2x0 F. (about 43-49xc2x0 C.), and then to finally disperse entirely during the final rinse cycles. The composition comprises a binder or matrix that desirably dissolves sparingly in water at a temperature less than 120xc2x0 F. (about 49xc2x0 C.) and preferably at less than 110xc2x0 F. (about 43xc2x0 C.), and readily at a temperature of about 155xc2x0 F. (68xc2x0 C.), and which binder is also preferably a surfactant, together with a chelating agent, which composition when dissolved in water provides a pH below 6. For best results, the tablet must dissolve only slightly during the initial rinse cycle or cycles, and then disperse substantially or entirely during the wash cycle, with however preferably a small residual amount surviving until the final rinse cycle or cycles, although the composition will perform adequately even if it completely disperses during the wash cycle.
The dishwasher cleaner tablet can be placed in the dishwasher and run through a regular dishwasher cycle when the dishwasher is empty, or it can be used in conjunction with conventional dishwasher detergent when the dishwasher is loaded with dishes, although the latter type of operation is not preferred as the dishwasher detergent may result in an increase in pH above 6 whereby cleaning of the dishwasher interior will be inhibited.
The present invention will be better understood with reference to the following detailed description of exemplary embodiments thereof.
As indicated above, the composition of the present invention includes two components which are most important, namely a matrix or binder material which will substantially survive the initial rinse cycles and having a dissolution or melting point sufficiently low so that it will largely or substantially disperse during the wash cycle and at least at a temperature of 155-160xc2x0 F. (about 68-71xc2x0 C.), and a chelating agent capable of attacking the inorganic deposits from the interior surfaces of the dishwasher machine, the composition in its dissolved form providing a cleaning solution having a pH lower than 6. In its simplest form, and ignoring the presence of optional other ingredients, the binder or matrix material may be present in an amount of 5% to 95% by weight, with the chelating agent being present in an amount of 95% to 5% by weight, more preferably 15%-70% binder and 30%-85% chelant, and most preferably 25%-40% binder and 75%-60% chelant.
Other ingredients are also desirably present in minor amounts, e.g. colorants, fragrances and preservatives and/or bactericides, preferably in an amount of no more than about 1% by weight of each based on the total solid composition. Other optional ingredients may also be added such as wetting agents and corrosion inhibitors, desirably in amounts no greater than about 5% by weight based on the total weight of the solid composition. More important is the provision of an antifoam agent in an amount of up to 20% of the solid composition, preferably about 1% to about 5% by weight based on the total weight of the solid composition. In addition, depending on the selection of the chelating agent and the matrix material, and whether or not the composition is designed for use with a dishwasher detergent, a small amount of a preferably solid acid may also be present to ensure that upon desolution the pH will be no greater than 6.
The function of the chelating agent is to carry out what is known as xe2x80x9cchelation solubilizationxe2x80x9d. From what appears above, it will be clear what properties are required for the chelating material, i.e. it must be capable of chelating at least the calcium ion, but preferably also the magnesium and iron ions; it is preferably an acid, or at least must be able to maintain chelating activity in an acid environment; and it should be only sparingly soluble at temperatures below about 43xc2x0 C. to about 49xc2x0 C., and more completely soluble at higher temperatures. If the chelating agent is not itself a solid at ambient temperatures, then it must be sufficiently compatible with the matrix or binder material so that the mixture thereof is solid at ambient temperatures and meets the aforementioned temperature dissolution requirements.
Preferred chelating materials are EDTA, citric acid, NTA, lauroyl ethylene diamine triacetic acid, oxalic acid, potassium bisulfate or EDTA variants. Mixtures of such chelating agents can also be used. Many chelating agents are known and commercially available and may be easily routinely tested for suitability according to the present invention; a list of chelating agents may be found, for example, in the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 5, Fourth Edition (1993), pages 764-795. Chelating agents for routine testing for suitability for use in the present invention may also be found in the patent literature.
The release controlling matrix or binding material can be any one of a number of solid water soluble solid materials having the property of dissolving slowly in water at temperatures below 140xc2x0 F. (60xc2x0 C.) and dissolving more quickly at temperatures above 150xc2x0 F. (about 65.6xc2x0 C.), and which preferably also have surfactant properties. These include alcohol ethoxylates, e.g. polyoxyethylated alcohols of preferably 16-20 carbon atoms, polyethylene glycol, polyvinyl pyrrolidone, polyvinyl acetate/pyrrolidone copolymers, N-acyl-N,N,N-ethylene diamine triacetic acid, etc. Other water soluble solid materials can be easily routinely tested for suitability for use in the present invention. In the case of the water soluble solid surfactants, the water solubility arises due to the suitable matrix solids having a melting point between about 60xc2x0 C. and about 71xc2x0 C.
If the chelating agent is an acyl ethylene diamine triacetic acid such as lauroyl ethylene diamine triacetic acid, then the matrix material cannot be the same material, i.e. an N-acyl-N,N,N-ethylenediamine triacetic acid. When such a material is used as the chelating agent, then at least 5% by weight of the composition should comprise another binder material to improve the dissolving rate and/or act as a stronger binder at temperatures below 140xc2x0 F.
As regards the optional ingredients, the wetting agents may be selected from a wide variety of surface active agents including water-soluble surfactants such as synthetic anionic, non-ionic, cationic, amphoteric and zwitterionic surfactants and mixtures thereof.
If an acid other than the chelating agent is incorporated in the solid composition of the present invention, it also is desirably a solid at room temperature, or at least is an acid which is fully compatible with the binder or matrix material in the quantities utilized to maintain the desired acid pH. Acids which are solid at ambient temperatures are well known.
The tablet of the present invention may be formed either by casting a melt of the components or by compression molding of thoroughly mixed powders according to well known techniques. For the typical sized home dishwasher, the size of the tablet is suitably 10-17 grams, but other sizes are also possible.
The present invention will be further understood from a consideration of the following illustrative examples which are intended to be purely exemplary, and not limitative.